Quantitative method for the production of single three-dimensional crystals from the vapor



United States Patent 3,527,623 QUANTITATIVE METHOD FOR THE PRODUCTION OF SINGLE THREE-DIMENSIONAL CRYSTALS FROM THE VAPOR Michael M. Schieber, Jerusalem, Israel, assignor to Massachusetts Institute of Technology, Cambridge, Mass., a corporation of Massachusetts N0 Drawing. Filed Feb. 19, 1968, Ser. No. 706,695 Int. Cl. B01j 17/28 US. Cl. 148--1.6

ABSTRACT OF THE DISCLOSURE Large single-crystal growth is accomplished by evaporating a specific amount of source material and condensing its vapor on a conical collector located at a specified distance from the source material where the supersaturation and temperature difference between the source material and collector are optimum for growth of large single crystals. Typical values are: three grams of sourcematerial per cubic centimeter of crystallization volume; a value between 0.1 and 10.0 for the supersaturation; and a ten-centimeter distance between the source material and the collector for a one-centimeter diameter tube.

Claims The invention herein described was made in the course of a contract sponsored by the Air Force Office of Scientific Research.

BACKGROUND OF THE INVENTION Field of the invention This invention pertains to a method for growing single crystals of high purity from polycrystalline materials and more particularly to a quantitative method for growing large single crystals from the rare earth metals.

Description of the prior art Growth of single crystals from the vapor generally involves a transport of vapor from a region containing a polycrystalline source material at a temperature T to a second region of crystal growth at a temperature T that is slightly below T thereby creating a condition of supersaturation 0' that, when properly controlled, can yield good crystal nuclei. The general procedure is to continuously pump the material or to seal the material into a tube that is either evacuated, placed in a protective atmosphere, or filled with an inert gas. The tube is then heated in a temperature gradient with the source material at the higher temperature. The vapor from the source material etfuses throughout the tube and condenses on the colder walls. The supersaturation cr of the vapor in contact with the crystal growing at T is given by (P -P )/P where P is the vapor pressure that corresponds to the temperature T and P is the vapor pressure that corresponds to the temperature T Prior methods of growth have been classified according to the means of producing the temperature gradient between the source material and the growing crystal. The temperature gradient is an important variable to be controlled because, although a sharp gradient at the point of supersaturation increases the rate of growth of the crystal, it usually produces a polycrystalline crystal rather than the desired single crystal. Single crystals are generally formed from a system near equilibrium, with a relatively small temperature ditierence between the source material and the crystal. The prior art utilizes the following three methods for producing a temperature gradient:

(a) a linear temperature gradient (b) a constant temperature diflference between the source material and a growth chamber (0) a cooled collector plate "ice In all of the above methods the temperature gradient is used to further supersaturate the material being grown. However, there has been no means for establishing a temperature T for a given compound that provides for the proper growth rate to induce large single crystal growth; nor has there been means for preventing crystals from spreading out along the sides of the tube wall which makes control of the gradient and consequently control of growth rather imprecise. As a result crystals that have been grown from the vapor have generally been small single crystals or poly-crystals.

SUMMARY OF THE INVENTION In the present invention about three grams per cubic centimeter of polycrystalline source material at a temperature T that can produce a vapor pressure P of 10 to 10- atmospheres without decomposing and which is confined in a crystallization tube is placed in a hot zone of a sealed or continuously evacuated vacuum furnace at the temperature T so that vapor is transported from the hot zone to a second region along the tube at a temperature T which corresponds to a pressure P The vapor is deposited on a metal-conical or frustum-conical shaped collector, the tip of which is placed at the region where the temperature T corresponds to a pressure P such that 0' has a value between 0.3 and 6.0. The tip of the collector is separated from the source material by a distance represented by the parameter L/D, greater than 7, where L equals the distance and D equals the diameter of the tube between the region of maximum deposition and the source material. Single crystals are grown during 24 to 48 hours of deposition followed by a very slow cooling process at a rate of about centigrade/hour in order to avoid strains in the crystal.

The crystals may be conveniently grown in either a standard horizontal or vertical furnace that is resistance or radio-frequency heated, provided that the temperature profile of the furnace allows the source material to be separated from the collector by the required distance. The crystals may also be grown in either a closed or continuously pumped system. In the case of the latter, a Knudsen cell may be used but is not necessary. The collector, which is the substrate upon which the material is grown, and the vessel containing the source material may be made of materials such as tantalum, tungsten, graphite, platinum, or molybdenum. Source materials that require a temperature T less than 1100 C. are preferably grown in a crystallization tube made of silica; those that require a temperature greater than 1100 C. are preferably grown in one made of either molybdenum or tantalum protected from outside oxidation by an external muflle through which inert gas is flushed.

The invention has the advantage of utilizing simple, standard, inexpensive apparatus for crystal growth. The invention further produces larger single crystals that are pure in structure, and enables the growth of metals such as the light rare earth metals that have heretofore not been grown by any other method.

The invention is particularly applicable to the singlecrystal growth of rare-earth metals such as europiurn, thulium, ytterbium, samarium, and dysprosium.

It is a principal object of the invention to provide a quantitative method for growth of any crystal from a source material that can produce a vapor pressure of 10" to 10- atmospheres without decomposing. It is a feature of the invention to use a conical collector not only to prevent multiple nucleation but also to confine the greater part of the vapor therein, thereby minimizing the tendency of crystals to spread out along the sides of the tube wall. It is a still further feature to locate the collector at a distance from the source material Where the supersaturation and temperature difierence between the source material and collector are optimum for growth of large single crystals from the source material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Large single three-dimensional crystals may be deposited or grown upon the collector in the following manner. A sutficient amount of a pure polycrystalline source material at a temperature T that can produce a vapor pressure of 10 to 10- atmospheres without decomposing is placed within a tantalum open-top container at one end of a silicia crystallization tube. The end of the tube containing the source material is then placed in a hot zone continuously evacuated by a vacuum furnace which may have a plurality of independently controlled regulated heating elements. The hot zone may be independently adjusted, by regulating the power input to the elements located at the hot zone to a temperature T which corresponds to a pressure P of 10- to 10- atmospheres to bring the source material to growing temperature. The source material is first evaporated without a conical collector causing vapor to be transported and condensed as a polycrystalline film over the entire length of the tube. The region along the tube at which maximum deposition occurs is then ascertained. The temperature T at the region of maximum deposition is measured by any convenient means such as a thermocouple, and the region may be maintained at that temperature by one of the independently controlled regulated heating elements. The supersaturation of the vapor at the temperature T is then calculated. An important parameter, represented by L/D where L equals tht distance and D equals the diameter of the tube between the region of maximum deposition and the source material, is next determined. If L/D is less than seven, the temperature profile of the furnace is adjusted by removing or adding electrical heaters along the tube length. A new quantity of source material is evaporated in a cleaned tube after the temperature profile has been adjusted to ascertain a new region along the tube at which maximum deposition occurs, thereby establishing a new temperature T a new value of supersaturation, and a new value for L/D. The process of varying the temperature profile and determining the new region of maximum deposition is repeated until the supersaturation a has a value between 0.3 and 6.0 and the parameter L/D is greater than seven. The tip of a tantalum conical collector is then placed at the region along the tube that will satisfy the aforesaid requisite conditions for L/D and which region is established as the growth region for the source material. The temperature T is maintained at a substantially constant temperature over the region at which the source material is located by means of one of the independently controlled regulated heating elements to avoid unwanted depositions at unwanted sites. Preferably three grams per cc. of source material are placed in the hot zone and evaporated for at least 24 hours to allow deposition upon the collector. Nucleation will occur either in the tip or at the wall of the collector and grow laterally outward from the wall until the collector is filled with crystal. By withdrawing the crystal up the tube at a rate equal to the deposition rate of the vapor, the crystals can be induced to grow vertically thereby providing for continuous deposition of vapor.

An alternate method for single crystal growth according to the invention is to seal an evacuated tube with the source material after the collector has been placed at the established growth region. Source material is then grown under similar conditions as for the above-described continuous pumping method.

The following examples, utilizing the above-described continuous pumping method, illustrate single crystal growth according to the invention.

4 EXAMPLE I Single crystals of samarium metal are grown if: at least 30 grams of polycrystalline samarium metal are placed in a crystallization tube having a length of 10 cm. between the source material and the tip of the collector and a diameter of 1 cm.; T is equal to 900 C. and T is equal to 840 C. in a vacuum of 10- mm.; o' is equal to 2.40; the vapor is deposited upon the collector for 24 to 48 hours followed by a very slow cooling process at a rate not more than C. per hour in order to avoid strains in the crystal; the contents are removed from the crystallization area after the furnace has reached room temperature and the vacuum has been discontinued. Size of the crystals grown is approximately 25 mm.

EXAMPLE H Single crystals of ytterbium are grown if T =615 C., T =585 C., and o'=0.75 under conditions similar to those of Example I. Size of the crystals grown is 5 mm.

EXAMPLE III Single crystals of europium metal are grown if T 720 C., T 630 C., and 0'=I.10 under conditions similar to those of Example 1. Size of the crystals grown is 2 mm.

EXAMPLE IV Single crystals of thulium metal are grown if T 1000 C., T =910 C., and 0:0.85 under conditions similar to those of Example I. Size of the crystals grown is approximately 2 to 3 mm.

EXAMPLE V Single crystals of dysprosium metal are grown if T =l220 C. and T =9l0 C. The temperature T which is greater than 1100 C., requires that the crystallization tube be made of either molybdenum or tantalum protected from outside oxidation by an external muflie through which inert gas is flushed. Otherwise, the conditions are similar to those of Example I. Size of the crystals grown is approximately 2 mm.

EXAMPLE VI Single crystals of silver metal are grown if T =950 C. and T =900 C. under conditions similar to those of Example I. Size of the crystals grown is approximately 3 mm.

It should be understood that if other configurations besides a cylinder are used for a crystallization tube, the term D will refer to a mean diameter of such configuration.

Although all of the above examples were grown with the source material separated from the tip of the collector by the parameter L/D of 10, a 0 within the range of 0.1 and 10.0, and approximately three grams per cc. of crystallization volume, inventor believes that single crystals can be grown if L/D is greater than seven, or a has a value within the range of 0.3 and 6.0, or the source material comprises approximately one gram per cc. of crystallization volume.

What is claimed is:

1. In a method for growing large single three-dimensional crystals from the vapor, the method being of the type wherein a polycrystalline source material, which, at a temperature T produces a vapor pressure P of 10- to 10 atmospheres without decomposing, and which is confined in a crystallization tube, is placed in a hot zone of a vacuum furnace at the temperature T so that vapor is transported from the hot zone and is condensed on the walls of a metal conical collector along the tube at a temperature T which corresponds to a pressure P the improvement which comprises:

(a) placing the tip of the collector at a distance, represented by the parameter L/D, greater than seven where L equals the distance between the hot zone and the tip of the collector and D equals the diam- 5 eter of the tube and at a temperature where the supersaturation has a value between 0.1 and 10.0;

(b) inserting in the hot zone an amount of source material sufficient to nucleate at and fill the collector;

(c) maintaining the temperature T at a substantially constant temperature over the length of the hot zone; and

(d) confining the greater part of the vapor within the collector by obstructing the transport of vapor be yond the collector, the vapor being condensed for a suflicient time to allow deposition upon the collector and cooled very slowly to avoid strains in the crystal.

2. The method as recited in claim 1, wherein placing the tip of the conical collector at a distance, represented by the parameter L/D, greater than seven comprises the steps of:

(a) evaporating the source material without the conical collector, the vapor of which is transported and condensed as a polycrystalline film over the entire length of the tube to ascertain the region of the tube at which maximum deposition of vapor occurs;

(b) measuring the temperature at the region at which maximum deposition of vapor occurs thereby ascertaining the value of supersaturation;

(c) measuring the distance L between the region at which maximum deposition of vapor occurs and the hot zone;

(d) adjusting the temperature profile of the furnace,

thereby varying the region at which maximum deposition of vapor will occur;

(e) repeating the process of steps (a), (b), (c), and

(d) until L/D is greater than seven and the supersaturation has a value between 0.1 and 10.0; and

(f) placing the tip of the conical collector at the location established in (e).

3. The method as recited in claim 1, wherein a sufficient amount of source material comprises three grams per cubic centimeter of crystallization volume between the source material at the hot zone and the tip of the conical collector.

4. The method as recited in claim 1, wherein a sufficient amount of source material comprises one gram per cubic centimeter of crystallization volume between the source material at the hot zone and the tip of the conical collector.

5. The method as recited in claim 1, wherein a suflicient time to allow deposition upon the substrate material is 24 to 48 hours.

6. The method as recited in claim 1 wherein the conical collector is withdrawn at a rate equal to the deposition rate of the vapor, thereby allowing a continuous deposition of vapor at the same value of supersaturation.

7. The method as recited in claim 1, wherein the supersaturation has a value between 0.3 and 6.0.

8. The method as recited in claim 1, wherein the parameter L/D is 10.0.

9. The method as recited in claim 1, wherein the polycrystalline source material is selected from the group consisting of the rare earth metals.

10. The method as recited in claim 1, wherein the polycrystalline source material is samarium.

11. The method as recited in claim 1, wherein polycrystalline source material is ytterbiurn.

12. The method as recited in claim 1, wherein polycrystalline source material is dysprosium.

13. The material as recited in claim 1, wherein the polycrystalline source material is europium.

the

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14. The method as recited in claim 1, wherein the polycrystalline source material is thulium.

15. The method as recited in claim 1, wherein the polycrystalline source material is silver.

References Cited UNITED STATES PATENTS 1,450,464 4/1923 Thomson 23294 2,754,259 7/1956 Robinson et al. 204-192 2,813,811 11/1957 Sears 1481.6 2,836,524 5/1958 Brenner et al. 148l.6

OTHER REFERENCES H. E. Nigh, A Method for Growing Rare Earth Single Crystals, Journal of Applied Physics, vol. 34, pp. 3323- 4, 1963.

L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner 

